Preventing the development of sourness in certain petroleum liquids during distillation



Patented May 20, 1952 UNITED STATS PATENT FF ICE- PREVENTING THE DEVELOPMENT OF SOUR- NESS IN CERTAIN PETROLEUM LIQUIDS DURING DISTILLATION No Drawing. Application April 25, 1950, Serial No. 158,088

11 Claims.

This invention relates to a method for the prevention of sourness during the fractionation of hydrocarbons which have, been previously sweetened or which contain small amounts of organic disulfides or other sulfur compounds. More particularly, the invention relates to the production of high end point naphthas which are free from deleterious mercaptans.

It is an object of this invention to prevent the formation of mercaptans and other deleterious sulfur compounds in hydrocarbon liquids containing organic disulfides and/or other sulfur compounds during the heating of said hydrocarbon liquids.

It is another object of this invention to provide a method for preventing the development of sourness during the distillation of doctor sweet hydrocarbons.

A still further object of the invention is to provide a process for making doctor sweet naphthas without the necessity of after treatment.

Certain hydrocarbon products and fractions which have been sweetened by any of the various conventional commercial sweetening processes wherein mercaptans are converted to disulficles, frequently yield fractions which are sour to the doctor test upon distillation. Among these commercial sweetening processes may be mentioned Doctor sweetening, the Linde process (oxidation with oxygen in the presence of copper chloride), the Perco process (oxidation with air in the presence of copper chloride) and air oxidation in the presence of a phenolic catalyst. Whether these hydrocarbon products have been sweetened or not, if they contain mercaptans, or disulfides which are decomposable to mercaptans on the application of heat, or other sulfur compounds which break down or distill under the conditions of fractionation, they will yield sourness on distillation or heating. This sourness (other than mercaptans originally present) is due mainly to the conversion by heat of the disulfides and possibly other sulfur compounds into mercaptans or sour-producing sulfur compounds. The extent of this conversion of disulfides into sour-producing mercaptans is a function of the temperature and the structure of the disulfides and is exceptionally high when the residence time in the fractionating unit is long or when the still charge is exposed to high skin temperatures. These skin temperatures have been detected to be as high as 600 to 650 F.

In the production of hydrocarbon thinners and solvents from various crude oil fractions, esp

cially from naphtha fractions, it is especially im-- portant that the end products be free from deleterious sulfur compounds and odors appurtenant thereto. It is equally important that a high yield of product be obtained since the close fractionation to produce these specialty products for the paint, varnish. and rubber industries is expensive and time consuming. The usual practice is to conduct such fractionations in specially designed rerun fractionation units from which an overhead stream and several closely controlled side streams can be obtained. These units usually comprise several fractionating units in series, each tower having a large number of plates.

For example, Linde sweetened long rangenaphtha stock used in the production of naphthol spirits and thinners when distilled has the following tests:

API Recovery Temperature F.) Chav- IBP 5% 10% 20% 30% 40% 50% 00% 05% El.

Twenty-eight per cent of the distillate product (above 290 F.) from this distillation is' doctor sour, having a mercaptan sulfur content of approximately 0.0009%. For certain of these fractions to be useful as a thinner or solvent they must pass the rigid requirement of having a merc'aptan sulfur content of less than approximately 0.0001 to 0.0002%. Therefore, it is necessary to subsequently treat these fractions with a solution containing from about 4% to 9% free caustic soda and from 0.2 to 0.3% sodium hypochlorite. This is usually followed with a water wash treatment.

This procedure represents the present practice and even though carefully controlled often presents problems of removal of remaining small amounts of sulfur compounds by subsequent retreatment or in some instances even redistillation or re-run to gain a product acceptable by the consumers.

According to the present invention, if the distillation of sweetened hydrocarbons is carried out in the presence of a small amount of glycerine or glycerine-water solutions, the aforementioned diiliculties are eliminated. The hydrocarbon fractions so produced contain less mercaptans and the higher boiling fractions are more easily obtained having acceptable properties. Hydrocarbon fractions produced in accordance with this invention pass the very severe Full Flask Corrosion Test. The Full Flask Corrosion Test corn prises conducting a distillation of the naphtha in accordance with A. S. T. M. D8646 except that a polished piece of copper is maintained upright in the liquid undergoing test and the distillation is carried out to the usual end point. During the test, the higher boiling disulfides present in the hydrocarbon accumulate in the last drop of liqiud present and decompose, giving a discoloration .of the piece of copper. Any disulfides which distill over, of course have very little efiect on the color of the copper strip. A passable Full Flask Corrosion Test is represented by a copper strip which has retained essentially its original copper color. A brass colored strip is termed barely passable for most purposes, and a gray or black copper strip is, of course, reiected. The reason for the test is to determine the presence of any and all corrosive sulfur compounds which may interfere with any operations in which the naphtha is used. This Full Flask Corrosion Test also gives an indication of the relative corrosiveness to be expected when the naphtha solvent is subjected to solvent recovery operations in copper ap aratus.

Our experiments have indicated that not only does the practice of our invention allow the product on of sweet and non-corrosive naphtha fractions in good yields, but it also provides a means for substantially eliminating the disulfid sulfurcontent from the naphtha fractions by leaving them behind in the residue. In former methods, if the naphtha distillates were sour or did not pass the Full Flask Corrosion Test, a subsequent sweetening step only served to convert the mercaptans to disulfides and did not remove them from the fractions. With the present process, the high molecular Weight disulfides accumulate in the last fractions without serious decomposition and all previous cuts are substantially free from deleterious sulfur compounds.

In order to further demonstrate the invention, the following examples are given:

Example 1.-Three hundred milliliters of a Linde sweetened long range naphtha stock, having a boiling range of between 256 and 438 F., containing approximately 0.03% of sulfur in the form of disulfide in addition to sulfur compounds other than mercaptans and disulfides, was distilled from a one liter glass fiask. Conditions of distillation were adjusted to give considerable residence time of the naphtha charge in the flask. Samples of the distillate were collected at regular intervals and given the doctor test. These samples began to show sourness to the doctor test when the temperature of the liquid in the flask was 350 to 363 F.

Example 2.--Several samples of a Linde sweetened long range naphtha. stock as used in Example 1 were each distilled under the same conditions except that from 0.09 to 0.18% by weight of glycerine was added to the naphtha charge at the beginning of each distillation. Again, samples were collected and tested. It was found that the distillate did not show sourness to the doctor test until the temperature of the naphtha in the flask was 384 to 394 F.

Example 3.-Three hundred milliliter of a Linde sweetened long range naphtha stock, as used in Examples 1 and 2, were distilled under the same conditions except that 0.18% by weight of glycerine was present in the form of approximately 50% aqueous solution. Upon the collection of samples and their subjection to the doctor test, sourness was found to appear when th temperature of the naphtha in the flask reached 382 F. It was noted that most of the water had distilled from the glycerine before this temperature was reached.

Example 4.Three hundred milliliters of the same Linde sweetened long range naphtha stock was distilled under the same conditions as in the previous examples except that 0.18% by weight of glycerine was'added at the beginnin of the distillation and water was added periodically during the distillation. On testing samples of the distillate as before, sourness was detected only when the temperature of the naphtha in the flask reached 406 F. No odor of acrolein was detected.

Example 5.Another distillation of the Linde sweetened naphtha was conducted under the same conditions as the previous examples except that no glycerine was added and water was injected periodically throughout the distillation. The collection of samples of distillate and their subsequent doctor testing and comparison with samples distilled in accordance with Example 1 disclosed that water was without effect on the development of sourness in the distillate.

Example 6.A Linde sweetened long range naphtha feed stock boiling between 256 F. and l38 F. and containing approximately 0.03% total sulfur, as disulfides and other sulfur compounds, was fractionally distilled on a plant scale using two efficient fractionating columns arranged so that a 295 to 330 F. fraction was taken overhead from the second column. The fraction contained 0.0009 to 0.0013% mercaptan sulfur and was doctor sour. When 0.0003 to 0.00045% glycerine (based on the volume of naphtha charge) was injected into the feed to the first column, the 295 to 330 F. fraction taken overhead from the second column contained only 0.0005% mercaptan sulfur. When this cut (taken during the glycerine injection) was sweetened in the plant with sodium hypochlorite, it was superior with respect to the Full Flask Corrosion Test to the corresponding out taken without glycerine injection. Furthermore, such a cut was much easier to sweeten .due to its lower mercaptan sulfur content.

From the above examples, it is clearly pointed out that the addition of a small amount of glycerine has had a dual effect upon the distillation. First, it is possible to produce a material of higher end boiling point without appreciable mercaptan formation, using glycerine, and secondly, the presence of mercaptans in the distillates has been either prevented altogether in the case of the lower cuts or considerably reduced in the case of the higher cuts. Example 6 shows that even under extreme conditions of local overheating, prevalent in plant scale operations, distillates are obtained which are more easily sweetened and more non-corrosive than those obtained in the absence of glycerine. The point of the entire operation is to prevent the decomposition of unstable disulfides and to accumulate the disulfides in the residue, thus yielding improved products when treating sweetened naphthas.

In order to demonstrate further the unique 'capacity of glycerine, in preventing the development of sourness during the distillation of stocks containing disulfides and organic sulfur compounds other than mercaptans, a series of distillations was made testing other compounds related to glycerine as possible inhibitors of mercaptan formation. These results are tabulated in Table I.

TABLE I Effect of additives on the development of doctor sourness daring distillation of Linde treated naphtha charging stock Temp. at Which Composite No. Additive Used Sourness First Progress of Sourness ml. from each Appeared cut) F. 1 None 363 Gradual to definitely sour at Sour.

373 F. All cuts sour from 373 to E. P. 400 F. 5m]. diethylene glycol.... 350 G%g;1%1 to definitely sour at Barely sweet. 5 ml. triethylene glycol 350 (35%? to moderately sour at Faintly sour. drops diethanolamine.. 358 Ggailgl? to definitely sour at Barely sweet. 5 ml. glycerine 400 Only slightly sour at 400 F Sweet. do None Still sweet at 414 F Do.

4dropsdisalicy1al propylene 340 Gradual to definitely sour at Moderately sour.

diamine. 365 F. 5ml. polyethylene glycol.. 358 Ggaggiafil to definitely sour at Faintly sour. 10 drops glycerine 388 to moderately sour at Sweet.

. do 394 Gradual to definitely sour at Do.

406 F. 5 drops glycerine 384 Do. 10 drops glycerine 2 drops 406 Only borderline at 406 F Do.

HQtO added after every 3 on s. l3 10 drops glycerine 10 382 Gradual to definitely sour at Do.

drops H2O. 395 F.

The results of the runs shown in Table I point up the unique character of glycerine as a means for preventing sourness in the fractions produced from a Linde treated naphtha charge stock. The diethylene glycol, triethylene glycol, diethanolamine and polyethylene glycol were ineflfective in attaining the desired result. This is shown in runs 2, 3, 4, and 8, wherein the initial temperature at which sourness appeared was from 5 to 13 F. lower than when the charge stock was distilled with no inhibitor present. The disalicylal propylene diamine used in run number '7 seemed to promote the formation of sourness under the conditions of the experiment. In the balance of the runs, except number 1, in which no additive was present, the glycerine raised the initial temperature at which sourness appeared on an average of from 19 to 51 F. depending slightly on the amount of glycerine present and the technique of its addition.

Additional experiments were conducted on a 300 ml. sample of naphtha stock (B. R. 306- 428 F.) with 10 drops (or about 0.5 ml.) of glycerine. The distillation was conducted in a 3-neck flask. The sample with added glycerine was brought to initial boiling temperature (about 306 F.) and a series of cuts taken each of ml. volume. After each three cuts, the flask was cooled and 2 drops of water added, then the distillation was continued. After the ninth cut, the volume of the cuts was reduced to 12.5 ml. The cuts were given the doctor test. The results ap- TABLE IIContinued (COOLED AND 2 DROPS WATER ADDED) ATER ADDED) Sweet.

Do. Borderline.

Slightly sour. Sour. 1

It is apparent from the results shown in Table II that the step-wise addition of water during the distillation has appreciably raised the end-' boiling point attainable before a sour distillate was produced. The amount of water added at any one time should not exceed the amount of glycerine present.

The proportion of glycerine used in accordanee with our invention is dependent on the amount and types of sulfur compounds present in the sweetened naphtha'to be treated. Also, within practical economic limits, the proportion of glycerine used will be dependent on the result desired. If too small an amount of glycerine is present during the distillation, there will be a reduction in its beneficial efiect and if an excess of glycerine is present, no harm is done nor added result attained except the economic loss of the excess. In general, about .00003 to 0.5% by weight of glycerine gives satisfactory results. In conducting distillations, the glycerine can either be added to the naphtha charge being distilled, or before distillation, or it may be injected either into the still pot or at various places along the fractionating column.

The action of the glycerine in preventing the formation of sourness during the distillations is not known. At temperatures of 290 to 295 C., glycerine has a tendency to form diand poly glycerols. Our experiments have shown that if the glycerine polymerizes or dehydrates, it loses its effectiveness, therefore, it is desirable to have atleast a small amount of water present during the distillation to prevent such reactions. This is important when treating higher boiling naphthas. Subsequent water wash of the naphtha fractions removes most of the by-products of the glycerine. If necessary, the last trace of these by-products can be removed by treatment with hypochlorite solution which oxidizes any acrolein present to acrylic acid, the latter being easily removed by caustic present in the hypochlorite solution.

Various parafiin-naphthene and intermediate base crudes may be used to produce sweet naphthas in accordance with this invention. Any crude which yields a variety of boiling range fractions having versatile solvent characteristics is considered as a good source of special naphtha products. Among the aliphatic naphthas which are specialized petroleum products contemplated within the meaning of this invention may be mentioned petroleum ether, B. R. 90 to 140 F.; iso-heptane, B. R. 166 to 196 F.; special textile spirits, B. R. 180 to 210 F.; iso-octane, B. R. 220 to 243 F.; close out V. M. & P. naphtha, B. R. 230 to 300 F.; light mineral spirits, B. R. 220 to 330 F.; Stoddard solvent, B. R. 310 to 385 F.; and high flash cleaning solvent, B. R. 360 to 400 F. The invention is not to be limited by this enumeration, the products mentioned being only exemplary thereof.

The glycerine (glycerol, C3H5(OH)3) used in accordance with this invention may be any of the following grades: dynamite, high gravity, refined, U. S. P., or C. P. The purity of the glycerine used will depend on the extent .of contamination allowable in the distillate products. Yellow grade glycerine is to be avoided since the contaminants therein are carried over and discolor the products. Pure glycerol has a specific gravity of about 1.2647 at 15 C., a refractive index of 1.4758 at 125 C., and B. P. of 209 C. A glycerine having these properties is entirely suitable for the present process.

What is claimed is:

1. In the method for preventing sourness in sweet products derived from petroleum hydrocarbons containing organic sulfur compounds during heating thereof to temperatures at which said compounds are converted into sour-producing compounds, the improvement comprising conducting said heating in the presence of a small amount of glycerine. 2. The method, in accordance with claim 1, in which the glycerine is present in an amount of 0.0003 to 0.5 per cent by weight based on the petroleum hydrocarbon.

3. The method, in accordance with claim 1, in

8 which the .glycerine is present in the form of an aqueous solution containing at least about 40 per cent by weight of glycerine 4. The method, in accordance with claim 1, in which a small amount of water is present during the heating.

5. The method of distilling a sweetened petroleum naphtha, containing organic disulfides which are converted into mercaptans under distillation conditions, to obtain naphtha fractions substantially free of said mercaptans, comprising heating said sweetened petroleum naphtha, under distillation conditions, in the presence of a small amount of glycerine, and removing naphtha fractions therefrom.

6. The method, in accordance with claim 5, in which the amount of glycerine present during the heating is from about 0.0003 to 0.5 per cent by weight based on the weight of petroleum naphtha '7. The method, in accordance with claim 5, in which a small amount of water is present during said heating.

8. The method, in accordance with claim 5, in which the glycerine is present in the form of a glycerine-water solution containing at least about 40 per cent by weight of glycerine 9. The method, in accordance with claim 5, in which the sweetened petroleum naphtha has a boiling range between about 250 to 440 F. under standard conditions.

10. The method of distilling a sweetened petroleumnaphtha, having a boiling range of about 250 to 440 F. under standard conditions, and containing organic disulfides which are converted to mercaptans at temperatures above 250 F., to obtain naphtha fractions having a mercaptan sulfur content of not more than about 0.0001 per .cent by weight, comprising heating said sweetened petroleum naphtha under distillation conditions, in the presence .of from about 0.0003 to 0.5 per cent by weight of glycerine, based on the weight of said sweetened petroleum naphtha.

11. The method, in accordance with claim 10, in which an amount of water equivalent to the amount of glycerine is present during the distillation.

GEORGE W. AYERS. MARCELLUS J. GEERTS. ROBERT E. CHANDLER.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,710,063 Morrell Apr. 23, 1929 2,297,866 Benedict Oct. 6, 1942 2,345,449 Birkhimer Mar. 28, 1944 

1. IN THE METHOD OF PREVENTING SOURNESS IN SWEET PRODUCTS DERIVED FROM PETROLEUM HYDROCARBONS CONTAINING ORGANIC SULFUR COMPOUNDS DURING HEATING THEREOF TO TEMPERATURES AT WHICH SAID COMPOUNDS ARE CONVERTED INTO SOUR-PRODUCING COMPOUNDS, THE IMPROVEMENT COMPRISING CONDUCTING SAID HEATING IN THE PRESENCE OF A SMALL AMOUNT OF CLYCERINE. 